1. Field of the Invention
This invention relates to rotary cone bits used for subterranean drilling and, more particularly, to inserts used with rotary cone bits that are specially engineered having a functionally graded polycrystalline diamond microstructure to provide improved elastic properties, mechanical properties and/or thermal properties when compared to conventional polycrystalline diamond inserts.
2. Background Art
Polycrystalline diamond (PCD) materials known in the art are formed from diamond grains or crystals and a ductile metal binder and are synthesized by high temperature/high pressure processes. Such material is well known for its mechanical properties of wear resistance, making it a popular material choice for use in such industrial applications as cutting tools for machining, and subterranean mining and drilling where such mechanical properties are highly desired. For example, conventional PCD can be provided in the form of surface coatings on, e.g., inserts used with cutting and drilling tools to impart improved wear resistance thereto.
Traditionally, PCD inserts used in such applications are formed by coating a carbide substrate with a layer of PCD. Such inserts comprise a substrate, a surface layer, and often a transition layer to improve the bonding between the exposed layer and the substrate. The substrate is typically a carbide material, e.g., cemented carbide, tungsten carbide (WC) cemented with cobalt (WC—Co).
The PCD layer conventionally includes metal binder up to about 30 percent by weight. The metal binder facilitates diamond intercrystalline bonding, and bonding of diamond layer to the substrate. Metals employed as the binder are often selected from cobalt, iron, or nickel and/or mixtures or alloys thereof and can include metals such as manganese, tantalum, chromium and/or mixtures or alloys thereof. However, while higher metal binder content typically increases the toughness of the resulting PCD material, higher metal content also decreases the PCD material hardness and wear resistance, thus limiting the flexibility of being able to provide PCD coatings having desired levels of hardness, wear resistance and toughness. Additionally, when variables are selected to increase the hardness or wear resistance of the PCD material, typically brittleness also increases, thereby reducing the toughness of the PCD material.
Conventional PCD inserts may include one or more transition layers between the PCD layer and the substrate. Such transition layers include refractory particles such as carbides in addition to the diamond and metal binder to change materials properties through the layers. However, carbide content manipulation does not always promote the best transition between adjacent PCD insert layers, permitting discrete interfaces to exist between the layers which can promote unwanted stress concentrations. The existence of these discrete interfaces, and the resulting stress concentrations produced therefrom, can cause premature failure of the PCD insert by delamination along the layer-to-layer interfaces.
It is, therefore, desired that a PCD insert be constructed in a manner that provides a desired balance of hardness, wear or abrasion resistance, and toughness while also reducing and/or eliminating the existence of residual stress concentrations within the construction to thereby provide an extended service life. It is also desired that the PCD insert be constructed in a manner that provides an improved degree of thermal stability during operation when compared to conventional PCD inserts, thereby effectively extending service life.